This invention relates to a process for using a phosphinocarboxylic acid and the salts thereof to dissolve and remove accumulation of silicate metal scale in aqueous systems. More specifically, this invention provides a method wherein certain phosphinocarboxylic acids and the metal salts thereof effectively dissolve and prevent the formation of silicate scale and sludge deposits in various aqueous systems.
It is generally known that the formation of scale and sludge deposits e.g. magnesium silicate scale on heating surfaces is one of the most serious problems encountered in aqueous systems such as in boiler operations, cooling towers, heat exchangers and the like. To prevent the formation of silicate scale, particularly magnesium silicate scale, from accumulating in these water systems, various treatments of the water generally requires the use of a combination of precipitating or dispersing agents, sludge conditioners and the like to keep the sludge in the systems fluid to be effectively removed. Aqueous systems which operate at high temperatures such as boilers, heat exchangers and the like are notoriously susceptible to the buildup of scale due to the impurities in the water e.g. calcium salts, iron oxides and particularly, magnesium salts such as magnesium silicates. The buildup of scale on the heat transfer surfaces is a serious problem primarily because the rate of heat transfer through the surfaces contributes to an overall loss of efficiency and therefore increased energy costs. Moreover, heat transfer surfaces with scale and sludge coatings are susceptible to corrosion beneath the scale since corrosion control agents are unable to effectively protect the metal surfaces.
Experience has proven that the use of various sophisticated water pre-treatment compositions do not always avoid the buildup of scale, particularly magnesium silicate scale in the water systems. Presently, scale prevention compositions include chelants, polymeric dispersants and conditioners or sequestering agents. Chelants, for example, e.g. sodium salt of ethylenediaminetetraacetic acid (EDTA) or the sodium salt of nitrilotriacetate (NTA) stoichiometrically are used to sequester metal ions such as calcium, magnesium, iron, etc. The removal of magnesium scale or sludge e.g. magnesium silicate is generally accomplished by descaling the systems with acid treatments. Acid treatments generally include the use of various organic acids such as citric or sulfamic or one of the mineral acids such as hydrochloric. However, it is obvious that in acid descaling, the system must be shutdown, drained, acid cleaned, rinsed, drained and then re-treated. Acid scaling therefore results in loss of time due to the shutdown which requires interrupting production schedules in order to achieve cleaning. Accordingly, a purpose of this invention is to provide a method of descaling by using comparatively large amounts of a phosphinocarboxylic acid or the salt thereof while the system is in continuous operation, therefore not requiring any shutdown or draining. It was found that effective amounts of the phosphinocarboxylic acids and its salts either alone or with other known dispersing agents, chelants and the like, are capable of dissolving silicate scale and sludge, particularly magnesium silicate scale that builds up in a water system and can be removed in a continuous process without interrupting the operation of the system.
Presently, various water-soluble polyelectrolytes and polymeric compositions are being used for controlling scale. The mechanism is primarily by prohibiting the formation of scale deposition by crystal modification and dispersion. Originally, the acrylic and maleic acid compounds, for example, were effectively used against calcium carbonate, the sulphates, etc. More recently, a number of high performance copolymers and terpolymers were developed which are also capable of controlling the buildup of multiple scalants including the magnesium and calcium silicates, phosphates, etc. Presently, however, there is no method available wherein any of these polyelectrolytes and/or polymeric compositions can be used to dissolve magnesium silicate scale once it has formed in the system.
For example, heretofore a process of inhibiting magnesium scale formation in an evaporation system, i.e. the processing of saline water, required the use of various polymeric materials such as a polyanionic compound derived from unsaturated diabasic acid and unsaturated sulphonic acid in combination with polycationic polymers. Desalination is an evaporation process wherein the units are operated under a vacuum either at atmospheric pressure or higher. In operating these desalination units, the temperature depends on the operating pressures which influences the nature of the scale that forms in the system. While scale formation caused by calcium carbonate can be controlled, for example, by the use of various additives, the problem of magnesium silicate scale or sludge control in evaporation systems has not been satisfactory. The rapid formation of magnesium silicate scale or sludge substantially reduces the efficiency of the system requiring a shutdown to remove the scale from the units. The loss of the operating time and the difficulty in removing the scale adds to the total cost of the operation.
In view of these problems, different methods have been proposed using certain chelating agents such as the polyphosphates as shown in U.S. Pat. No. 2,782,162. In addition, polyelectrolytes such as the sodium polyacrylates were found to be effective to prevent calcium carbonate scale at high temperatures. U.S. Pat. No. 3,981,779 teaches the use of various chelants and surfactants, e.g. aminodiacetic acids and their salts to decrease the buildup of magnesium hydroxide scale to a limited degree. Further, U.S. Pat. No. 3,985,671 teaches the use of alkylene phosphates and quaternary ammonium chloride compounds as scale control agents for recircling water in cooling systems. Thus, while a number of chelants and polymeric compounds have been used to inhibit the formation of scale in the operation of evaporation and heat transfer systems, etc. none of these compositions were found to be effective in dissolving and removing magnesium scale, i.e. magnesium silicates from these systems.